Refactor signature caching and navigation logic

Replaced the previous signature persistence mechanism with
`IMemoryCache` for temporary storage of captured signatures
using a unique `Guid` key and a 1-minute TTL. Added logging
to track cached signatures and their associated envelope keys.

Removed the logic for rebuilding and displaying reports with
overlaid signatures. Instead, implemented navigation to a
new signed page (`/envelope/{EnvelopeKey}/signed`) with the
signature ID passed as a query parameter.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Components/Pages/EnvelopeReceiverReportPage.razor

@@ -10,12 +10,14 @@
 @using DevExpress.Blazor
 @using System.Drawing
 @using System.Security.Claims
+@using Microsoft.Extensions.Caching.Memory
 @inject NavigationManager Navigation
 @inject IJSRuntime JSRuntime
 @inject EnvelopeGenerator.Server.Client.Services.AuthService AuthService
 @inject EnvelopeGenerator.Server.Services.EnvelopeReceiverAuthorizationService ReceiverAuthorizationService
 @inject EnvelopeGenerator.Server.Services.EnvelopeReceiverPageDataService PageDataService
 @inject AppVersionService AppVersion
+@inject IMemoryCache MemoryCache
 @inject ILogger<EnvelopeReceiverReportPage> Logger
 @implements IDisposable
 
@@ -678,32 +680,19 @@
         };
         _signaturePopupVisible = false;
 
-        // Persist to cache (fire-and-forget)
-        if (_receiverUser is not null)
-        {
-            _ = Task.Run(async () =>
-            {
-                try { await PageDataService.SaveCachedSignatureAsync(_receiverUser, _capturedSignature); }
-                catch { /* non-critical */ }
-            });
-        }
+        // Store signature in IMemoryCache with a Guid key (1 minute TTL)
+        var sid = Guid.NewGuid().ToString("N");
+        MemoryCache.Set(
+            sid,
+            _capturedSignature,
+            TimeSpan.FromMinutes(1));
 
-        // Rebuild the report with signatures overlaid
-        if (_pdfBytes is not null)
-        {
-            var newReport = BuildReport(_pdfBytes, _signatures, _capturedSignature);
+        Logger.LogInformation(
+            "Signature cached with sid={Sid} for envelope {EnvelopeKey}", sid, EnvelopeKey);
 
-            if (_reportViewer is not null)
-            {
-                await _reportViewer.OpenReportAsync(newReport);
-                // Dispose previous report after opening new one
-                _report?.Dispose();
-            }
-
-            _report = newReport;
-        }
-
-        await InvokeAsync(StateHasChanged);
+        // Navigate to signed page
+        Navigation.NavigateTo(
+            $"/envelope/{Uri.EscapeDataString(EnvelopeKey!)}/signed?sid={sid}");
     }
 
     async Task<string?> GetActiveSignatureDataUrlAsync()